1. Field of the Invention.
The present invention relates to semiconductor materials, and, more particularly, to growth of II-VI compound semiconductors materials.
2. Description of the Related Art.
Alloys of mercury telluride and cadmium telluride, generically denoted Hg.sub.1-x Cd.sub.x Te, are extensively employed as photosensitive semiconductors for infrared radiation. Indeed, Hg.sub.0.8 Cd.sub.0.2 Te has a bandgap of about 0.1 eV which corresponds to a photon wavelength of 12 .mu.m and Hg.sub.0.73 Cd.sub.0.27 Te a bandgap of about 0.24 eV corresponding to a photon wavelength of 5 .mu.m; and these two wavelengths are in the two atmospheric windows of greatest interest for infrared detectors. In particular, extrinsic p-type Hg.sub.1-x Cd.sub.x Te has potential application in infrared focal plane arrays operating in the 10-12 .mu.m wavelength window.
The Hg.sub.1-x Cd.sub.x Te is either formed as a bulk crystal or grown on a substrate such as CdTe by liquid phase epitaxy or other methods in order to have a more convenient substrate. Such epitaxial Hg.sub.1-x Cd.sub.x Te films typically have a thickness in the order of ten .mu.m.
Molecular beam epitaxial (MBE) films of Hg.sub.1-x Cd.sub.x Te alloys are currently grown with a substrate temperature of approximately 200.degree. C., onto a substrate which, before the growth of the Hg.sub.1-x Cd.sub.x Te film, presents a surface of CdTe. The crystal orientation of the CdTe is either (111)B or (100). Currently, the orientation preferred is the (100), which eliminates twinning which is present in the (111)B orientation. See, J. Arias et al, 5 J. Vac. Sci. Tech. A 3133 (1987). Other orientations are also used. The carrier concentration of the material is set in one of two ways. In the first, the growth conditions, that is, the substrate temperature and beam fluxes, are adjusted to give the desired carrier concentration, with the material being used in the as-grown conditions (n type is grown at about 190.degree. C. and p type at about 210.degree. C.). In the other method, which has only recently been reported, after growth the material is given an anneal in mercury saturated conditions to annihilate the acceptor states which arise from metal vacancies in the as-grown material and convert p type to n type. This anneal is performed at a low temperature, but less than 300.degree. C., for above this temperature, there is a large concentration of metal vacancies which are stable. Indeed, annealing at over 300.degree. C. will convert as-grown n type material to p type.
However, this growth procedure has the problem of producing material with anomolous electrical properties.